Calibrating, Printing and Proofing by the G7™ Method

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Calibrating, Printing and Proofing by the G7 Method Version 6, August 2006 Appendix A: Production Press Control NOTE: This process assumes all separations, plates and proofs have been made in an ICC workflow based on standard Characterization Data and G7 calibration. The G7 press control method is similar to conventional press control, with CMYK solid-ink readings (in CIELab or equivalent densities) as a starting point, followed by mid-tone readings to monitor tone shape. The difference is that traditional TVI readings are replaced with absolute ND (or Lab) 'HR' readings, and traditional Print Contrast calculations are replaced with absolute ND (or Lab) 'SC' readings. This approach monitors gray balance and lightness more reliably than TVI readings, and with less chance of mid-tone density or gray balance variations, however because gray balance is typically less stable than TVI values, be prepared for wider deviations throughout the run. Note also that this method of press control is quite new, with no well-established history of acceptable tolerances. Be prepared to experiment over time to establish the optimum tolerances for your equipment, conditions and expectations (see Appendix G). A.1 Run press up to nominal ink levels Begin by running the press to specified ink colorimetric values (see Appendix H) or SIDs. Remember that CIELab values take priority over density values. Once make-ready is complete, RELATIVE densitometry (based on reading a good sheet with your densitometer) remains a valid and efficient basis for run-time press control, and the same density goals can be used to make-ready future runs – at least for that specific paper/ ink/ densitometer combination. A.2 Adjust gray balance, HR_cmy and SC_cmy Adjust solid ink densities or other press variables within permitted tolerances till the HR_cmy patch (50c, 40m, 40y) is as close as possible to the a* and b* gray balance aim values, and ND or L* aim value. Also check that the CMY SC patch (75c, 66m, 66y) is as close as possible to the SC_cmy aim value. Some compromise will usually be necessary due to normal press variations, but try and share any error evenly between HR and SC patches. A.3 Adjust HR_k and SC_k Adjust black solid ink density or other press variables till the K HR patch (50k) is as close as possible to the nominal HR_k aim. Also check that the SC_k patch (75c, 66m, 66y) is as close as possible to the nominal SC_k aim value. Some compromise will usually be necessary due to normal press variations, but try and split any error evenly between HR and SC patches. A.4 Nominal HR, SC and HC aim values The chart below shows nominal aim values for HR, SC and HC for paper with a 95 L* value. These values assume the new G7 50k < 50,40,40cmy convention introduced in February 2006. Gray Minus Paper (ND) With Paper (ND) With Paper (L*) Patch CMY K CMY K CMY K 25% (HC) 0.25 0.22 0.31 0.28 75.7 77.7 50% (HR) 0.54 0.50 0.59 0.55 57.5 59.9 75% (SC) 0.90 0.90 0.96 0.96 39.5 39.7 Nominal gray patch aim densities and L* values for GRACoL #1 grade (ISO Type 1 & 2) paper NOTE: If you prefer the 2005 G7 specification with same NPDC for K and CMY, use the CMY aim values for black, and/or the CMY graphs in A.5, but remember this is not acceptable in official GRACoL 7 or SWOP implementations, and will darken the black plate slightly more than ‘normal’. 38 Copyright © International Digital Enterprise Alliance, Inc. [IDEAlliance] (2006). All Rights Reserved.
Calibrating, Printing and Proofing by the G7 Method Version 6, August 2006 A.5 HR, SC and HC aim values for other dynamic ranges Aim values for other printing types depend on the dynamic range of the device, as shown below. Note that while SC (75% curve) varies continuously as a function of Device Density Range, HR (50% curve) is approximately constant for typical offset Density Ranges (1.3 and higher), and HC (25% curve) is approximately constant for density ranges of 0.7 or higher. Graph showing HR, SC, and HC aim values for devices with different dynamic ranges NOTE: If you prefer the same NPDC for K and CMY, take black aim values from the CMY graphs. A.6 Maintaining gray balance by CIELab Throughout the run the same L*a*b* values should be maintained on the CMY gray and 50K patches. If gray balance and SIDs move in a similar direction, small SID adjustments (see chart in 6.4) should restore gray balance. A.7 Maintaining gray balance by CMY density Another way to control gray balance is by recording the C, M and Y densities of a balanced CMY gray patch at make-ready, then maintain the same readings throughout the run. This approach may not be reliable for make-ready, because of densitometer variations, but it is sufficient for run-time control because ink adjustments are directly indicated by CMY density variations. Left: A typical in-tolerance gray balance reading in Lab. Right: The same gray patch measured in CMY density mode, showing typical equivalent densities – (both with paper included). CAUTION: The sample readings illustrated above are for illustration purposes only, and do NOT represent actual aim values. 39 Copyright © International Digital Enterprise Alliance, Inc. [IDEAlliance] (2006). All Rights Reserved.
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Calibrating, Printing and Proofing by the G7™ Method

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